This invention relates generally to gas turbine engine carburetors and more specifically to such carburetors effective for utilizing relatively low BTU gaseous fuels.
Recent advances in coal and biomass gasifier technology have generated a growing interest in airblown gasifier gas turbine engine systems for use in both stationary and mobile power generation applications. Depending on the nature of the coal or biomass material supplied to the gasifier and on the gasifier type, gas fuels having relatively low heating values of about 3.72 to about 5.58 megajoules per standard cubic meter (MJ/m.sup.3), or, equivalently, about 100 to 150 BTU per standard cubic foot (BTU/SCF) is produced (hereinafter referred to as "low BTU" gaseous fuel or, simply, fuel).
Marine and Industrial (M&I) gas turbine engines are typically derived from aircraft gas turbine engines which conventionally utilize carburetors effective for mixing air with an atomized liquid fuel having relatively high BTU values. Two examples of conventional gas turbine engine carburetors which atomize a liquid fuel for mixing with air are disclosed in U.S. Pat. No. 4,180,974--R. E. Stenger et al and U.S. Pat. No. 3,853,273--D. W. Bahr et al, both assigned to the present assignee and incorporated herein by reference.
An M&I engine is typically substantially identical to the aircraft engine from which it is derived. However, some M&I engines are structurally modified to operate on alternative fuels such as, for example, natural gas. Natural gas has a heating value of about 850 to about 900 BTU/SCF which is less than that of liquid fuels which have a relatively higher BTU content. To utilize natural gas, a conventional atomizing type fuel injector in the carburetor is not needed or usable, and a relatively simple gas injector is typically substituted therefor.
A known gas fuel injector includes a gas delivery tube and a simple, hollow injector tip having a plurality of apertures facing directly in a downstream direction along the centerline of a typical combustor. For a relatively large M&I engine, the tip outlet apertures may be predeterminedly sized for providing a sufficient volume flow rate of natural gas for combustion. Of course, inasmuch as the basic M&I engine is substantially identical to its corresponding aircraft engine, the volume flow rate of natural gas must be, accordingly, proportionately higher than the flow rate of the liquid fuel required for the designed-for power requirements of the engine to account for the relatively lower BTU content of natural gas as compared to the liquid fuel.
However, it has been discovered that for relatively small combustors found in small aircraft derivative M&I engines, the use of a conventional natural gas injector is unacceptable. For example, in view of the relatively high fuel volumetric flow rate required to meet the design requirements of the engine, the space restrictions of the conventional swirler assembly used in the carburetor of the combustor place a limit on the maximum flow area size of the fuel injector tip outlet apertures and results in relatively high discharge velocity of gases therethrough. This discharge velocity would be so high that the gas could not properly mix with air from the swirler, but instead would be directed as a jet through the combustor without properly being mixed and burned therein.